ILS Localizer 2

28.10.2004

Submission:

The azimuthal coverage for the ILS Localizer is addressed in both contributions. I agree fully that in most cases today the wide azimuthal coverage defined in Annex 10 (standard +-35°) is no longer needed. I don't know a minimum ILS-coverage of +-10° as claimed to be defined in Annex 10. The MLS azimuthal coverage was adapted to the ILS-requirements. However, the standard MLS-coverage of +-40° can be adapted by technical means very simply by adapting the scan-range, e.g. +20° and -10°. So MLS is much more flexible compared to ILS. Other navaids means, like GPS/GNSS and also the radar vectoring technique make the large disadvantageous coverage almost obsolete. I've proposed and discussed the reduction of the coverage on several international conferences in the past, but the only way is an explicit site dependant reduction to be published today.

Let me ask the procedure community about the required minimum coverage of the ILS-glide slope subsystem. Annex 10 defines +-8° but does not define uniquely the tolerances to be applied. This coverage is also independent of the type and length of the runway. On difficult sites, the specified azimuthal coverage may be difficult or impossible to achieve meeting also the standard positioning requirement of 120m offset for the mast. One way out is the endfire antenna supplied by WATSS antenna in extreme cases where forward truncations of the reflection plane play a major role also. Due to the standard intercept procedures for ILS, the coverage of +-8° is not needed having in mind in particular that the aircraft use the radar altimeter close to the runway. My proposal, also made on several conferences, is to reduce the coverage to +- 2 times the related Localizer width, i.e. about +-3.6° for a typical 3000m runway and a typical ILS installation. By this the coverage depends on the runway length. Long runways would have a narrower azimuthal coverage of the ILS glide slope. Have in mind that the single LOC-width yields a full indication of the CDI already. Quite a number of ILS glide slopes with a reduced azimuthal coverage are published, e.g. +-4°. The answers I got so far are expressing no problems. Are there any objections against such kind of proposal from the procedure point of view?

Dr.-Ing. Gerhard Greving - NAVCOM Consult